The middle seat slips around a phew rails- not on top, but physicailly
locked in place, but still alloxing +/- X movement.  To stop this
movement, there are bolts that go through rails on the seat and rails on
the phloor into nuts (but looks like threaded holes) welded into
phloor-boards of the bus.  THese bolts are either 8 or 10mm x 30. I'd go
to the hardware store and purchase 4 or 6 each or 8mm and 10mm (washers
too) and see what fits.

Hope this helps. Email me/us if you have continued problems.
CHeers
Matthew

Matthew Pollard             http://www.uidaho.edu/~poll7356
Dept. of Chemistry               http://www.chem.uidaho.edu
University of Idaho                   http://www.uidaho.edu